Hydrogen-air (H2-air) fuel cells are well known in the art, and have been proposed as a power source for many applications. In such fuel cells, hydrogen is the anode reactant (i.e. fuel), oxygen from air is the cathode reactant (i.e. oxidant), and water is the reaction product. The hydrogen is provided from a H2-source such as stored H2, or H2 formed by the reformation of a hydrogenous (i.e. hydrogen-containing) material such as gasoline or methanol. A plurality of individual cells are commonly bundled together to form a fuel cell “stack” which comprises a pair of end cells sandwiching a plurality of inboard cells therebetween.
There are several known types of H2-air fuel cells including aqueous-acid-type, aqueous-alkaline-type, and Proton-Exchange-Membrane-type (PEM). PEM fuel cells have potential for high power densities, and accordingly are desirable for motive-power/vehicular-propulsion applications (e.g. electric vehicles). PEM fuel cells include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton-transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on its opposite face. The membrane is typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode and cathode typically comprise finely divided catalyst particles admixed with proton conductive resin. The catalyst particles are often supported on carbon particles. The MEA is sandwiched between a pair of electrically conductive current collectors which contain a network of reactant flow channels therein defining a so-called “flow field” for distributing the H2 and air over the surfaces of the respective anode and cathode catalysts. The inboard cells are defined by bipolar such current collectors, often called “bipolar electrodes”. The end cells are defined by a bipolar electrode on one side (i.e. confronting the stack) and a cell end plate, often called a “monopolar electrode” on the other side (i.e. facing away from the stack). A pair of current-collecting terminal plates, one at each end of a fuel cell stack, engage the monopolar cell plates of the end cells to collect the current produced by the stack and direct it to an external electrical load (e.g. a propulsion motor) powered by the stack. Compression plates, on the extreme ends of the stack, outboard the current-collecting terminal plates, are attached either to side plates, or to tie-bolts, that extend the length of the stack, and serve to hold the stack together under compression. An insulating plate electrically insulates the compression plates from the current-collecting terminal plates.
The exothermic, current-producing electrochemical H2+O2→H2O reaction produces, product water in situ within the cell during the normal operation of the fuel cell. In the case of aqueous-acid or aqueous-alkaline fuel cells, this product water is taken up by the electrolyte, and hence does not freeze when the fuel cell is stored in a below-freezing environment. However, in a PEM fuel cell, the product water can freeze within the stack which (1) can plug/clog the reactant flow fields with ice, and prevent or restrict reactant gas flow, (2) can damage the polymer membranes, and (3) can exert deleterious pressures within the cells resulting from expansion of the water during freezing. Accordingly, it is known to dehydrate PEM fuel cells before storing them under freezing conditions. However, starting-up a frozen PEM stack still produces product water that can condense, freeze and damage and/or ice-clog the stack by blocking flow of the cell's reactants, especially in the flow-field and header/manifold regions near the current collectors which are particularly susceptible to ice-clogging. Even when ice-clogging is not an issue (e.g. in aqueous-acid/aqueous-alkaline fuel cells), poor performance from end cells, during cold start-up, prolongs the time it takes before the stack can generate full power. End cells perform worse than inboard cells because (1) the stack's current-collecting terminal plates are heat sinks that draw heat out of the end cells, and (2) there is only one MEA heating the end cells (i.e. in contrast to multiple MEAs heating the inboard cells) when drawing current from the stack during cold start.